This invention relates to a process for producing a rigid polyurethane foam having a remarkably low thermal conductivity and a low density with a short mold release time, and the rigid polyurethane foam thus produced.
Rigid polyurethane foams are usually obtained by reacting an polyol component with an isocyanate component in the presence of a blowing agent, a reaction catalyst and a foam stabilizer. Since rigid polyurethane foams are generally excellent in heat insulating properties, they are widely used as various kinds of heat insulating materials but have many problems in moldability, uniformity in foaming, and the like. Various proposals have been made in order to improve these problems.
For example, Japanese Patent Unexamined Publication No. 58-134108 proposes to use aromatic amine polyols mixed with a diluting polyol having a higher proportion of primary hydroxyl group and a low viscosity, e.g. a polyol having a molecular weight of 600 or less and obtained by adding ethylene oxide to a polyhydric alcohol, in order to produce rigid polyurethane foams with good moldability. Further, Japanese Patent Unexamined Publication No. 58-134109 proposes to use aromatic polyols terminals of which are capped with ethylene oxide as one component of polyol components in order to improve heat insulating properties by enhancing the uniformity of foam sizes of low density rigid polyurethane foams. But the rigid polyurethane foams obtained by the two above-mentioned processes of Japanese Patent Unexamined Publications have a relatively large thermal conductivity of 13.6 to 13.9.times.10.sup.-3 Kcal/m. hr. .degree.C., which values are insufficient as an heat insulating material for refrigerators.
As heat insulating materials for refrigerators, it is desired to have a thermal conductivity of 12.5.times.10.sup.-3 Kcal/m. hr. .degree.C. or less, more preferably 12.0.times.10.sup.-3 to 11.0.times.10.sup.-3 Kcal/m. hr. .degree.C., a low density and a mold release time of a short time, preferably 5.5 minutes or less, more preferably 5 minutes or less.
In order to meet the above-mentioned requirements, European Patent No. 91,828 discloses a process for producing a rigid polyurethane foam wherein there is used as a polyol component a polyol mixture comprising (1) a tetrafunctional tolylenediamine polyol, (2) a bifunctional propylene polyol, (3) an octafunctional sucrose polyol and (4) a trifunctional diethanolamine polyol. The rigid polyurethane foam obtained by this process has an average foam diameter of 0.3 to 0.5 mm, a foam density (a panel foam density) of as low as 28 to 30 kg/m.sup.3 and a mold release time of as short as 4 minutes, but a thermal conductivity is as slightly high as 13.0 to 15.0.times.10.sup.-3 Kcal/m.hr. .degree.C., which value is insufficient for refrigerators.
On the other hand, Japanese Patent Unexamined Publication No. 62-81414 discloses a process for producing a rigid polyurethane foam having an average foam diameter of as fine as 0.1 to 0.2 mm, and a thermal conductivity of 11.0 to 12.0.times.10.sup.-3 Kcal/m.hr..degree.C. by using a polyol mixture containing methyl glucoside polyol. According to this process, the thermal conductivity is remarkably improved, but undesirably the skeleton strength of foam is weakened, the foam density (a panel foam density) is as large as 33 to 36 kg/m.sup.3 (10 to 20% is increased compared with the value of European Patent No. 91,828) due to very fine foam density, and the mold release time is 6 minutes or longer.
Therefore, it has long been desired to produce rigid polyurethane foams having a very low thermal conductivity and a low density in a short mold release time, suitable for use as a heat insulating material for refrigerators.